So. Embassytown is a small human enclave on a backwater planet on the edge of the explored universe, reachable only by a very dangerous trek through the anti-matter of space. Arieka is home to the Ariekei, aliens that look sort of like a horse/coral reef/insect mashup. The Ariekei have a Language, but because of some bizarre quirks in their evolution, their use and understanding of Language is so vastly different from humans that no one can really communicate with them. The two-mouthed Ariekei speak a doubled language, and because there's no difference between the signified and the signifier, they're incapable of lying or using simile or metaphor, unless that simile or metaphor has taken place in the physical world.
Furthermore, the Ariekei can't communicate with humans, because their language requires a single mind to speak it's doubled words - and we can't. Enter the Ambassadors, cloned human-twins trained from birth to be as single-minded as possible in an attempt to speak to the Ariekei.
Confused yet? You will be, because Meiville chucks you into the narrative without so much as a by-your-leave, which is fine. Part of the genius of this work is that the mystery of the Ariekei unfolds for the characters the same way it does for the reader.
Anyway, Embassytown's heroine is Avice Benner Cho, an immerser who has returned to Embassytown with her linguist husband. Cho holds the honor of being a simile - the girl who ate what was given to her - in the Ariekei Language.
Embassytown putters along in its isolated, backwatery way, until the arrival of a new Ambassador pair who have been altered to speak with a single mind. Unfortunately, no one predicted the reaction of the Ariekei, who promptly become desperate junkies, hooked on the sound of the Ambassador pair's voice. Chaos descends on Embassytown, as the embattled Embassy splits into factions, all desperate to find a way to figure out how to save the Ariekei from their addiction - and save themselves from being murdered by an advancing Ariekei army.
Yes, Embassytown is bizarre,